Abstract

The first shock wave experiments performed on silicate materials were reported for quartz in 1962. The intervening forty years have allowed for extensive investigation of SiO2 by dynamic, static and theoretical means. Previous studies have concluded that quartz transforms completely to stishovite at ~40 GPa and melts at ~115 GPa along its Hugoniot. Recent discoveries that SiO2 transforms to phases slightly more dense than stishovite have led to a reexamination of the dynamic compression of SiO2 in this thesis. Based on comparing calculated Hugoniots to data for multiple initial SiO2 phases, it is proposed that, in addition to the stishovite and melt transitions, quartz is completely transformed to the CaCl2 structure at ~70 GPa. Coesite shows evidence of complete transformation to stishovite at ~ 50 GPa, and to the CaCl2 structure at ~65 GPa. Due to the higher temperature achieved in the quartz samples the slope of the stishovite-CaCl2 phase boundary is constrained to be ~180 K/GPa.
From a similar analysis of Hugoniot data collected for high quality MgSiO3 natural crystal and synthetic glass in this study, and existing data, it is concluded that along the crystal Hugoniot akimotoite is attained at ~70 GPa, perovskite structure at ~110 GPa and melt at ~ 170 GPa. It is found that the melt is 2-3 % denser than the solid at pressures greater than 100 GPa, after correcting for thermal differences in the two regimes. An important implication is a negative Clapeyron slope, leading to a decreasing melting temperature with increasing pressure, above ~ 100 GPa. These observations increase the possibility of the existence of a significant amount of partial melt in the lowermost mantle, e.g., the ultra low velocity zone.